The acquisition of data and subsequent generation of computer models for real-world objects is of interest in many industries, for applications including architecture, physical plant design, entertainment applications (e.g., in movies and games), surveying, manufacturing quality control, medical imaging, and construction, as well as cartography and geography applications. In order to obtain accurate models of an object, as well as the area in which that object exists in the real world, it is necessary to take accurate measurements or samplings of surfaces that make up the object and any elements of the surrounding area. Historically, this sampling was carried out by surveyors, photogrammetrists, or technicians using techniques that provided samples at the rate of tens or hundreds per hour at most. Since the amount of data was relatively small, the data was easily dealt with in standard, off-the-shelf CAD programs or other modeling software.
Recent advances in technology such as LIDAR scanning technologies have resulted in the ability to collect billions of point samples on physical surfaces, over large areas, in a matter of hours. In a LIDAR process, a laser beam scans across a view that encompasses the structure of interest. The scanning device measures a large number of points that lie on surfaces visible in the scene. Each scan point has a measured location in 3D space, to within some measurement error, that typically is recorded relative to a point (x, y, z) in the local coordinate system of the scanner. The resulting collection of points is often referred to as one or more point clouds, where each point cloud can include points that lie on many different surfaces in the scanned view.
LIDAR systems are described, for example, in U.S. Pat. No. 5,988,862, filed Apr. 24, 1996, entitled “INTEGRATED SYSTEM FOR QUICKLY AND ACCURATELY IMAGING AND MODELING THREE DIMENSIONAL OBJECTS,” which is hereby incorporated herein by reference. An exemplary LIDAR system 100 shown in FIG. 1 utilizes a Field Digital Vision (FDV) module 102 that includes a scanning sensor for scanning an object 104, such as a building of a piece of machinery. The scanning sensor also can sense the position in three-dimensional space of selected points on the surface of the object 104. The FDV module 102 generates a point cloud 106 that represents the sensed positions of the selected points. The point cloud 106 also can represent other attributes of the sensed positions, such as reflectivity, surface color, and texture, where desired.
A control and processing station 108 interacts with the FDV 102 to provide control and targeting functions for the scanning sensor. In addition, the processing and control station 108 can utilize software to analyze groups of points in the point cloud 106 to generate a model of the object of interest 104. A user interface 116 allows a user to interact with the system, such as to view a two-dimensional (2D) representation of the three-dimensional (3D) point cloud, or to select a portion of that object to be viewed in higher detail as discussed elsewhere herein. The processing station can include any appropriate components, such as standard computer and/or processing components. The processing station also can have computer code in resident memory, on a local hard drive, or in a removable drive or other memory device, which can be programmed to the processing station or obtained from a computer program product such as a CD-ROM or download signal. The computer code can include instructions for interacting with the FDV and/or a user, and can include instructions for undertaking and completing any modeling and/or scanning process discussed, described, or suggested herein.
The FDV 102 can include a scanning laser system (LIDAR) 110 capable of scanning points of the object 104, and that generates a LIDAR data signal that precisely represents the position in 3D space of each scanned point. The scanning laser system can include a beam steering unit (not shown) for directing laser pulses along a scan path, such as by following a raster pattern over an area of an object to be scanned. The beam steering unit also can direct the reflected portion of each pulse back to a detector or transceiver of the LIDAR system. The LIDAR data signal for the groups of scanned points can collectively constitute the point cloud 106. In addition, a video system 112 can be provided, which in one embodiment includes both wide angle and narrow angle CCD cameras. The wide angle CCD camera can acquire a video image of the object 104 and provides to the control and processing station 108, through a control/interface module 114, a signal that represents the acquired video image.
The acquired video image can be displayed to a user through a user interface 116 of the control and processing station 108. Through the user interface 116, the user can select a portion of the image containing an object to be scanned. In response to user input, the control and processing station can provide a scanning control signal to the LIDAR 110 for controlling the portion of the surface of the object that should be scanned by the LIDAR. More particularly, the scanning control signal can be used to control an accurate and repeatable beam steering mechanism that steers a laser beam of the LIDAR 110. The narrow angle CCD camera of the video system 112 can capture the intensity of the laser returned from each laser impingement point, along with any desired texture and color information, and can provide this captured information to the control and processing station 108. The control and processing station can include a data processing system (e.g., a notebook computer or a graphics workstation) having special purpose software that, when executed, instructs the data processing system to perform the FDV 102 control and targeting functions, and also to perform the model generation functions discussed elsewhere herein. Once the object has been scanned and the data transferred to the control and processing station, the data and/or instructions relating to the data can be displayed to the user.
FIG. 2 shows a block diagram of an optical transceiver 200 of the FDV. The optical transceiver 200 transmits an optical pulse to a spot on an object (or structure) being scanned, and receives back an optical pulse reflected from the object. Given the constant speed of light, the optical transceiver calibrates the distance to the spot on the target. A laser 202 fires the optical pulse, which typically lasts less than 250 psec, in response to an external command provided from a laser controller 204. The laser 202 produces the pulse, at a wavelength such as about 532 nm, within about 100-300 microseconds after receiving a command signal. The command signal emanates from a digital signal processor that provides central control of real time events. The time delay is a function of variables such as laser age, recent laser history, and environmental/operating conditions.
The output of the laser 202 is transmitted through a beam expander 206 that is focused to adjust the size of a light spot that will eventually impinge upon a point on the object being scanned. The focused optical pulse then is transmitted through a duplexer 208, which is an optical system for aligning the outgoing optical path with the incoming optical path. The duplexer 208 directs a significant first portion of the light energy of the outgoing optical pulse to a spot on the object via a scanner 210. A second but much smaller portion of the light energy of the outgoing optical pulse is directed to a receiver telescope 212. The portion of the outgoing optical pulse that propagates to the object impinges on a spot on the object, and some of the energy of the optical pulse is reflected off the object in a direction back to the duplexer 208. The returning optical pulse is directed by the duplexer 208 to the receiver telescope 212, which focuses the received energy onto a detector 214. The detector 214 converts the received optical pulse energy into electrical energy. The output of the detector is a series of electrical pulses, the first (generated by the detector in response to the small portion of the transmitted pulse not directed toward the object) occurring at a short fixed time (i.e., fixed by the length of the optical path through the beam expander, duplexer, and receiver telescope) and the second occurring as light energy returns from the object. Both the second, small portion of the transmitted pulse and the return optical pulse reflected from the spot on the object are provided to the timing circuit 216, which calculates the time of flight to the spot on the object. The range to the spot on the object can then be readily calculated from the calculated time of flight.
FIG. 3 is a block diagram showing an exemplary laser device 300 of the FDV. The heart of the laser system 300 is a conventional laser chip 302 that includes two bonded crystals coated with antireflective dielectric coatings. The laser chip 302 is pumped with a solid state diode 304 operating at 808.5 nm±0.3 nm. The output frequency of the diode pump 304 is adjusted by changing the pump temperature with a thermoelectric cooler 306. The temperature of the diode pump 304 is measured with a thermistor 308, and the measured temperature is fed back into the diode power supply 310. The required temperature varies with each individual diode, but typically ranges from 20.degree. to 30.degree. C.
The output power of the diode pump 304 is typically 1 Watt, launched into a 100 micron core glass fiber. When continuously pumped, the output of the crystal laser 302 is approximately 35 mW average power at 1.064 microns, which corresponds to 2.4 microJoule pulses lasting about 280 psec at a repetition rate of 15 kHz. The multimode fiber is terminated by an SMA905 solid brass connector, with the crystal of the laser chip 302 glued to one end of the connector with an optical resin. This ensures adequate thermal dissipation from the crystal of the laser chip 302, keeping the crystal within the temperature range required for most efficient operation.
A piece of KTP frequency doubling crystal 312 is held within a few millimeters of the face of the laser chip crystal 302. This provides an ultimate output from the laser 300 having a 12 mW average power at 532 nm, which corresponds to 0.8 microJoule pulses lasting approximately one third of a nanosecond. This ultimate output from the laser 300 is nearly diffraction limited (i.e., one which has theoretically minimum divergence, given a specific wavelength and waist diameter), with an apparent waist diameter of 56 microns. The laser can meet FDA Class II eye safe system design specifications, where the maximum energy per pulse that can be transmitted at 532 nm is 0.2 microJoules. With this restriction, the average power transmitted is largely dependent upon the pulse repetition rate.
The performance of a LIDAR system can vary over time, as well as under differing environmental and/or operating conditions. The performance variations can include changes in the intensity of each laser pulse emitted from the LIDAR system, as well as the duration and relative timing of each of the pulses. These changes can increase the margin for error in each point sample collected. For high precision measurements, these the increased error margins can result in unacceptably imprecise results.
Further complicating matters from a technical standpoint is the fact that any surveying instrument utilizing a laser beam must meet stringent safety regulations. The current state of the art involves reducing the output power of the system in order to ensure the laser beam meets a particular laser classification. Previous scanning systems met class II US regulations by limiting the power of individual pulses, ensuring a minimum pulse width, and limiting the number of laser pulses per second, regardless of the range or other operating parameters of the scan. In this way the laser beam was ensured to be Class II at all times. These limitations can cause problems with laser measurements, however, as the intensity of the laser is reduced. A reduction in intensity can increase the margin for error in large-scale applications. It also can increase the difficulty in locating the laser spot using a camera of the scanner device. These restrictions also function to limit the use of the device in terms of scanning speed and single point measurements.